Polycarbonate and polyester films, which have conventionally been used as optical films can be easily provided with a function for imparting a retardation to transmitted light by stretching of the film. Although dependent on the final use of the film, in almost cases, this retardation imparting function was satisfactory. However, because the anisotropy of the refractive index is overly large, in other words, the values of the photoelastic coefficient (CP) and the stress optical coefficient (CR) are overly large, minor variations in stress can cause a large variation in the transmitted light retardation imparting function. Consequently, this retardation imparting function has suffered from problems of reliability and poor stability over time.
In the case of films of triacetylacetate, the anisotropy of the refractive index is comparatively smaller, but these films suffer from problems relating to heat resistance, and deformation on water absorption.
Cyclic polyolefin based resins have properties which include a high glass transition temperature due to the rigidity of the primary chain structure, a non-crystalline structure with a high light transmittance due to the presence of bulky groups on the primary chain structure, and a low birefringence due to the small anisotropy of the refractive index, and are consequently drawing considerable attention as transparent thermoplastic resins with superior heat resistance, transparency and optical characteristics. Examples of this type of cyclic polyolefin based resin are disclosed in Japanese Laid-open publication (kokai) No. 1-132625 (JP1-132625A), Japanese Laid-open publication (kokai) No. 1-132626 (JP1-132626A), Japanese Laid-open publication (kokai) No. 63-218726 (JP63-218726A), Japanese Laid-open publication (kokai) No. 2-133413 (JP2-133413A), Japanese Laid-open publication (kokai) No. 61-120816 (JP61-120816A), and Japanese Laid-open publication (kokai) No. 61-115912 (JP61-115912A).
In recent years, the utilization of the above properties in the application of cyclic polyolefin based resins to optical materials such as optical films, optical disks, optical lenses and optical fibers and the like, as well as to fields such as sealing materials for optical semiconductor sealing has been the subject of much investigation. In other words, the application of films formed from cyclic polyolefin based resins to a variety of different film applications have been reported as potential solutions to the aforementioned problems associated with conventional optical films. Specific examples include the retardation plates formed from cyclic polyolefin based resin films disclosed in Japanese Laid-open publication (kokai) No. 4-245202 (JP4-245202A), Japanese Laid-open publication (kokai) No. 4-36120 (JP4-36120A), Japanese Laid-open publication (kokai) No. 5-2108 (JP5-2108A) and Japanese Laid-open publication (kokai) No. 5-64865 (JP5-64865A). The application of cyclic polyolefin based resin films to protective films for polarizing plates is disclosed in Japanese Laid-open publication (kokai) No. 5-212828 (JP5-212828A), Japanese Laid-open publication (kokai) No. 6-51117 (JP6-51117A) and Japanese Laid-open publication (kokai) No. 7-77608 (JP7-77608A). Moreover, a liquid crystal display element substrate formed from a cyclic polyolefin based resin film is disclosed in Japanese Laid-open publication (kokai) No. 5-61026 (JP5-61026A).
In the applications described above, the fact that a cyclic polyolefin based resin with a water absorption of no more than 0.05% can be produced with relative ease, and this low water absorption value are reported as the special characteristics of the resins, and are described as essential properties. However, if this type of low water absorption cyclic polyolefin based resin film is used as a retardation plate or a liquid crystal display element substrate, then the film may suffer from inferior adhesion to a hard coat, an antireflective film or a transparent conductive layer, or inferior bonding with the polarizing plate and the glass. In cases in which a cyclic polyolefin based resin film is used as a protective film for a polarizing plate, then in addition to the adhesion problems outlined above, an additional problem arises in that the water of the water based adhesive typically used for bonding the film to the polarizer is very difficult to dry.
However, the cyclic polyolefin based resin family includes a wide range of different structures, and not all cyclic polyolefin based resins display a water absorption of 0.05% or less. In order to ensure a water absorption of no more than 0.05%, the cyclic polyolefin based resin must have either a polyolefin structure formed from only carbon atoms and hydrogen atoms, or a structure which incorporates a proportion of halogen atoms.
Consequently, in order to resolve the aforementioned problems relating to low water absorption, optical films incorporating a thermoplastic norbornene resin with a polar group incorporated within the molecular structure have been disclosed in Japanese Laid-open publication (kokai) No. 7-287122 (JP7-287122A) and Japanese Laid-open publication (kokai) No. 7-287123 (JP7-287123A). The optical films disclosed in these applications display superior optical characteristics including a high degree of transparency, a low retardation of transmitted light, and a uniform and stable application of retardation to transmitted light upon stretching and orientation, offer good levels of heat resistance and adhesion and bonding with other materials, and moreover also undergo little deformation on water absorption.
However, these conventional cyclic polyolefin based resins have a refractive index anisotropy which is overly small, in other words, the values of the photoelastic coefficient (CP) and the stress optical coefficient (CR) are too small, and so when an attempt is made to equip an optical film incorporating this type of cyclic polyolefin based resin with a function for imparting a retardation to transmitted light, there are restrictions on the conditions for stretching and orienting the film, and the transmitted light retardation imparting function may be insufficient. In other words, because obtaining transmitted light with a large retardation is difficult, then depending on the final use of the film, the function for imparting retardation to transmitted light may be insufficient.